1. Field of the Invention
This invention relates to a lenticular lens sheet for a projection screen used in what is called a transmission-type projection television (hereinafter the television is abbreviated as TV) in which a picture is projected to a screen from its back side and the picture transmitted through the screen is viewed on its front side. More particularly, it relates to a lenticular lens sheet used for HDTV projectors or LCD projectors which has a small lenticular pitch (i.e., one cycle of black stripes or light absorbing layers on the exit side; hereinafter simply called a pitch).
2. Description of the Related Art
In transmission screens used in transmission-type projection TVs, lenticular lens sheets are used for the purpose of expanding a visual angle in the horizontal and vertical directions.
The transmission-type projection TVs are commonly constituted of, as shown in FIG. 7, three cathode-ray tubes (CRTs) 9, 10 and 11, of red, green and blue, respectively, arranged side by side in a line; projection magnifying lenses 12, 13 and 14 through which pictures on the respective CRTs are enlarged; and a screen 17 comprised of a Fresnel lens 15 and a lenticular lens 16, on which an enlarged image is formed.
In such constitution, the angle (indicated by .epsilon. in FIG. 7) made by straight lines that connect the center of the screen and the center of each projection lens is usually 8.degree. or more, and the angles at which the light rays of the respective colors are incident on the screen 17 are different from one another. Hence, there are problems peculiar to projection TVs such that color tones may change depending on the horizontal position at which the picture on the screen is viewed and color tones may differ at every position on the screen at the fixed horizontal position. The former, a change in color tone, is called a color shift, and a state in which the latter color-tone difference is large is called a poor white uniformity.
In order to make smaller the color shift and improve the white uniformity, what has been conventionally used is a double-sided lenticular lens comprised of, as shown in FIG. 8, an entrance lens 18 comprising a cylindrical lens formed on the entrance surface, an exit lens 19 comprising a cylindrical lens also formed on the exit surface, and a light absorbing layer 20 formed at the light non-convergent part of the exit surface. In this instance, the entrance lens and the exit lens each usually have a shape of a circle, ellipse or hyperbola represented by the following formula (III): ##EQU2## (wherein C is a main curvature, and K is a conic constant), or have a curve to which a term with a higher order than 2nd order has been added.
In screens making use of such a double-sided lenticular lens 16, it is proposed to specify the positional relationship between the entrance lens 18 and exit lens 19 or the shapes thereof. For example, it is proposed to specify the positional relationship between the entrance lens 18 and exit lens 19 in such a way that the lens surface of one lens is present at the focal point of the other lens (Japanese Patent Applications Laid-open No. 57-81254, No. 57-81255, etc.). It is also proposed to specify the eccentricity of the entrance lens so as to be substantially equal to a reciprocal of refractive index of the material constituting the lenticular lens 16 (Japanese Patent Application Laid-open No. 58-59436). It is still also proposed to combine two sheets of double-sided lenticular lenses in such a way that the optic axis planes of the respective lenticular lenses fall at right angles with each other, and also to form such double-sided lenticular lenses in such a way that the entrance lens and exit lens at the periphery of one of the lenses are asymmetric with respect to the optic axis (Japanese Patent Application Laid-open No. 58-108523). In order to obtain a bright picture, it is proposed to make smaller the region of visual field in the vertical direction of a lens than the region of visual field in the horizontal direction thereof (Japanese Utility Model Publication No. 52-4932). There is also an example in which the position of light convergence only at the valley of an entrance lens is set aside toward the view side from the surface of an exit lens so that the tolerance for disagreement of optic axes and the difference in thickness can be made larger or the color shift can be made smaller (Japanese Patent Application Laid-open No. 1-182837).
Besides these methods in which the positional relationship between the entrance lens 18 and exit lens 19 or shapes thereof are specified, it is also common to uniformly disperse light-diffusing fine particles in the whole lenticular lens sheet so that the visual field angle in the horizontal direction can be ensured mainly by the refraction effect of the lenticular lens, and the visual field angle in the vertical direction, by the diffusibility of the fine particles.
However, to sufficiently ensure the vertical-direction visual field angle by dispersing the light-diffusing fine particles in the lenticular lens may bring about a problem that the picture is blurred because of the scattering of light due to the light-diffusing fine particles. For this reason, it is attempted to produce fine irregularities on the exit surface by various means so that the visual field angle in the vertical direction can be ensured. For example, there are proposals such that no exit lens 19 opposed to the entrance lens 18, as shown in FIG. 8, is formed on the exit surface of a lenticular lens, and beads with refractive index substantially equal to that of a lenticular lens are mixed into the surface of the lenticular lens (Japanese Patent Application Laid-open No. 63-163445), beads with refractive index substantially equal to that of a lenticular lens are mixed into a thermoplastic resin film materal to form a film having fine irregularities on its surface, and the film obtained is thermally contact-bonded to the exit surface of the lenticular lens (Japanese Patent Application Laid-open No. 1-161328), or the exit surface of a lenticular lens is roughed by sandblasting and abrading the inner surface of a mold used for shaping lenticular lenses (Japanese Patent Application Laid-open No. 3-43924).
As described above, various proposals have been hitherto made on lenticular lenses for the purpose of decreasing the color shift or white non-uniformity, brightening pictures and ensuring appropriate visual fields in both the horizontal direction and the vertical direction.
In all of the above lenticular lenses, however, it has been unsettled i) to more increase the transmission efficiency of light to make the picture brighter, and ii) in order to obtain a picture with a high resolution, to make the lens pitch of the lenticular lens very small while maintaining the lens thickness to a certain extent.
Here, increasing the transmission efficiency of light, which is the first-mentioned problem to be settled, has been questioned in the following way: As previously described, the shape of each entrance lens 18 and exit lens of the double-sided lenticular lens as shown in FIG. 8 is represented by the formula (III). In this instance, in order to make the incident light emergent in a good efficiency, the constant K must be -2.ltoreq.K.ltoreq.1, and preferably -0.8.ltoreq.K.ltoreq.0, and at the same time the light convergence must be improved so as for the light convergent point to be substantially a single point on the exit lens 19 over the whole width of the entrance lens 18 for each lens pitch.
Some conventional lenticular lenses have succeeded in controlling the conic constant K in the formula (III) to be within the above range. However, an attempt to make the light convergent point be substantially a single point on the exit lens 19 over the whole width of the entrance lens 18 has caused the problem that vertical streaks are seen when a screen is viewed from the vicinity of the horizontal visual field angle at which the luminance reasonably decreases in the horizontal direction. Such vertical streaks are caused by the shape uneveness that valleys 18a of the respective entrance lenses in the screen have shapes delicately different one another for the respective lenses. For this reason, in the conventional double-sided lenticular lenses, the shape of the valley 18a is so controlled that the light entering from the valley 18a of the entrance lens is totally reflected on the surface of the exit lens and can not be easily emergent from the exit lens. Hence, in the vicinity of this valley 18a, a lowering of the transmission efficiency of light has been brought about.
Making the lens pitch finer, which is the second-mentioned problem to be settled, has been questioned in the following way: In general, in the double-sided lenticular lenses, the visual field angle in the horizontal direction is substantially determined by the shape of the entrance lens and the thickness of the lenticular lens. Here, in order to make the half visual field angle in the horizontal direction not less than .+-.37.degree. which is required in usual screens when the light convergent point formed by the entrance lens is substantially in the vicinity of the surface of the exit lens, the thickness of the double-sided lenticular lens must be made not more than 1.1 to 1.3 times the pitch of the entrance lens. As for the projection TVs of the HDTV type having an excellent resolution, the pitch of the lenticular lens is required to be set very small, for example, not more than about 0.6 mm, in order to prevent the horizontal resolution from being adversely affected by a screen. Accordingly, at the pitch of 0.6 mm or less, the thickness of the lenticular lens is required to be made as mall as not more than 0.78 mm, in order to ensure 37.degree. or more for the half visual field angle in the horizontal direction. In the case of LCD projectors, the light non-transmitting portion (dark portion) of each picture element of a liquid crystal panel is so large that the moire tends to occur because of the arrangement of picture elements and the arrangement of lenticular lens sheets. In order to prevent such moire, the pitch of the lenticular lens is desired to be made much smaller, for example, not larger than about 0.3 mm. Hence, the thickness of the lenticular lens in this instance is required to be made much smaller.
In extrusion, which is the most efficient method for the mass-production of lenticular lenses, it is difficult to manufacture lenticular lenses with a thickness of 0.78 mm or less, because of a difficulty in the extrusion itself. Even if such extruded products can be obtained, there is the problem that the products tend to be broken.
Meanwhile, it is known that the extrusion can be stably carried out when the extruded product is in a thickness of about 0.9 mm or more. A product extruded in this thickness but in a pitch of not more than 0.6 mm, however, can give no lenticular lens having less color shift at a half visual field angle of 37.degree. or more in the horizontal direction.
To cope with this problem, some may contemplate that, in the lenticular lens like the above, having a thickness of about 0.9 mm or more and a pitch of 0.6 mm or less, the horizontal visual field angle attributable to the lens may be made a little smaller and the horizontal visual field angle corresponding to what has been made small may be compensated by the light diffusion properties of light-diffusing fine particles to obtain the desired large horizontal visual field angle. Such use of light-diffusible fine particles usually tends to bring about a decrease in the color shift, but may cause another problem that their use results in an increase in the light diffusion properties in the interior of the lenticular lens and also results in a decrease in the amount of light reaching to the exit lens itself and an increase in the amount of light reaching to the black stripe region of the exit lens side, to lower the transmission efficiency of light.